Preheat the oven. Preheat your oven to 180°C conventional (160°C fan bake). Lightly grease a 23–25 cm square baking dish.
Prepare the apples. Peel, core and slice the apples into approximately 5 mm thick slices. Weigh out 900 g of prepared apples and place them directly into the baking dish.
Make the apple filling. Add the brown sugar, lemon juice, cinnamon, vanilla extract and cornflour to the apples. Toss everything together until the slices are evenly coated. Spread the apples into an even layer and scatter the 30 g of cubed butter over the top.
Make the crumble topping. In a large mixing bowl, combine the flour, brown sugar, cinnamon and salt. Add the cold, cubed butter and rub it into the dry ingredients using your fingertips until the mixture resembles coarse breadcrumbs with a few larger buttery clumps.
Assemble the crumble. Sprinkle the crumble mixture evenly over the apples, making sure they're completely covered. Leave the topping loose rather than pressing it down so it bakes into a lovely crumbly texture.
Bake. Place the dish in the preheated oven and bake for approximately 45–55 minutes, or until the crumble topping is golden brown and the apple filling is bubbling around the edges. Check that the apples are tender by inserting a knife into the centre. If the topping browns too quickly, loosely cover with foil.
Rest and serve. Remove from the oven and allow to rest for 10–15 minutes before serving. Enjoy warm with vanilla ice cream, custard or cream.